Transform a load from an undef/zero global into an undef/global even if we
have complex pointer manipulation going on. This allows us to compile
stuff like this:
__m128i foo(__m128i x){
static const unsigned int c_0[4] = { 0, 0, 0, 0 };
__m128i v_Zero = _mm_loadu_si128((__m128i*)c_0);
x = _mm_unpacklo_epi8(x, v_Zero);
return x;
}
into:
_foo:
xorps %xmm1, %xmm1
punpcklbw %xmm1, %xmm0
ret
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@41022 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 7532643..2fad0ab 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -8782,6 +8782,28 @@
return false;
}
+/// GetUnderlyingObject - Trace through a series of getelementptrs and bitcasts
+/// until we find the underlying object a pointer is referring to or something
+/// we don't understand. Note that the returned pointer may be offset from the
+/// input, because we ignore GEP indices.
+static Value *GetUnderlyingObject(Value *Ptr) {
+ while (1) {
+ if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) {
+ if (CE->getOpcode() == Instruction::BitCast ||
+ CE->getOpcode() == Instruction::GetElementPtr)
+ Ptr = CE->getOperand(0);
+ else
+ return Ptr;
+ } else if (BitCastInst *BCI = dyn_cast<BitCastInst>(Ptr)) {
+ Ptr = BCI->getOperand(0);
+ } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) {
+ Ptr = GEP->getOperand(0);
+ } else {
+ return Ptr;
+ }
+ }
+}
+
Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
Value *Op = LI.getOperand(0);
@@ -8860,6 +8882,17 @@
return Res;
}
}
+
+ // If this load comes from anywhere in a constant global, and if the global
+ // is all undef or zero, we know what it loads.
+ if (GlobalVariable *GV = dyn_cast<GlobalVariable>(GetUnderlyingObject(Op))) {
+ if (GV->isConstant() && GV->hasInitializer()) {
+ if (GV->getInitializer()->isNullValue())
+ return ReplaceInstUsesWith(LI, Constant::getNullValue(LI.getType()));
+ else if (isa<UndefValue>(GV->getInitializer()))
+ return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
+ }
+ }
if (Op->hasOneUse()) {
// Change select and PHI nodes to select values instead of addresses: this